Field of the Invention
The present invention relates to a process and apparatus that recovers a helium rich stream from a mixed gas having low concentrations of helium therein. More specifically, the invention relates to an integrated process and apparatus for treating a mixed feed gas from an operating process that produces a liquid product from natural gas containing helium, such as processes that produce ammonia, methanol, or liquid hydrocarbons.
Description of Related Art
Helium is a rare gas, which has unique physical properties making it valuable in a wide range of commercial applications. For instance, helium is inert, its boiling and melting points are the lowest among the elements, it is the second lightest element, and it exists only as a gas in all but the most cryogenic conditions. Commercial applications range from balloons to lasers and include heat transfer and cooling applications; shielding applications; gas tracing applications; inert environments for growing crystals; and numerous other applications. However, due to its limited availability and increasing commercial demand, its costs have increased significantly over that the last few years.
While helium is the second most abundant element in the universe, it is extremely rare on Earth. In fact, it is a finite resource and is primarily found trapped with natural gas in concentrations up to about seven percent by volume although most often, it is found in concentrations of less than 0.5% by volume. Almost all of the commercial helium requirements are supplied by the extraction from helium-containing natural gas by low temperature fractional distillation processes. The resulting helium rich gases are further purified or refined using additional cryogenic distillation steps or by pressure swing adsorption (PSA) processes which selectively remove other gases. These final refining steps result in commercial grades of helium in excess of 99.9%.
The high cost and limited availability of helium gas has resulted in investigations to identify other sources of helium. The source most relevant to the present invention is hydrogen containing gas streams produced from or existing in various current commercial processes. Process gas streams such as those from the production of hydrogen, or which recycle hydrogen, can contain helium in concentrations which can be economically extracted and recovered. For example, processes for the production of synthesis gas (CO/H2), which is typically further used in hydrogen production, ammonia production or used in certain Fischer-Tropsch reactions, typically begin with natural gas feed streams. The various processing and recycling steps do not normally separate the inert helium already present from the natural gas feed stream and it often accumulates in the processing steps. The resulting recycle and purge gas can contain helium in concentrations of up to 2% or more by volume.
Processes using multi-step cryogenic distillation or hybrid PSA systems for removing or extracting helium from natural gas or other gases containing low concentrations of helium are known. These processes are typically directed to natural gas feed streams and require significant capital investments and complex and expensive cryogenic distillation systems to generate the enriched helium gas streams which can be further processed or integrated into the starting extraction process. Similarly, related processes for the recovery of helium from refinery off gases have been taught.
In U.S. Pat. No. 3,807,185, helium containing vent gas from the synthesis of ammonia is subjected to several partial condensation steps and then cooled to remove the higher-boiling components, namely, methane, nitrogen, and argon in a phase separator. A low temperature stripping column is then employed to recover liquid hydrogen and a gaseous helium/hydrogen mixture. Hydrogen and argon can also be recovered. This process requires multiple cooling and heating steps and, the hydrogen feed stream must be condensed at extremely low temperatures requiring high capital equipment costs and high power requirements.
U.S. Pat. No. 5,089,048 discloses a method for extracting helium from a relatively helium poor gas mixture through a pressure swing adsorption process which allegedly achieves an enriched product gas containing over 50% helium.
U.S. Pat. No. 7,871,577 discloses a gas-to-liquids process where a hydrogen-helium mixture is produced by cryogenic separation. Specifically, higher the molecular weight hydrocarbon compounds or oxygenates are produced from a gas comprising methane in a process including the steps of generating synthesis gas by reaction of a gas including methane with steam and/or an oxidant gas comprising oxygen, producing higher molecular weight hydrocarbon compounds or oxygenates in a syngas conversion process, removing offgas including unreacted hydrogen and unreacted carbon monoxide from the syngas conversion process and separating cryogenically unreacted hydrogen from the offgas or from a gas derived therefrom to produce separated hydrogen product that is substantially free of unreacted carbon monoxide and a first cryogenic liquid including unreacted carbon monoxide.
U.S. Pat. No. 8,152,898 discusses helium recovery from gas streams containing high concentrations of hydrogen gas and low concentrations of helium gas, such as from the recycle stream from the production of ammonia.
The related art discussed, above, does not address the recovery of a helium stream from an upstream process (e.g. ammonia plant or gas-to-liquids plant) where the offgas is processed through cryogenic separation in combination with one or more hydrogen transport membranes.
The mixed offgas from the upstream source generally contains hydrogen, which has a very low boiling point (i.e., similar to helium), and which hinders the separation process. One of the advantages associated with the present invention is that through the employment of a hydrogen transport membrane, in the system of the present invention, the hydrogen-helium separation is achieved.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.